Channel Modification

Over thousands of years people have made changes to the physical structure of the rivers, such as altering the rivers course and installing flood defenses and weirs. Many of these changes have benefited society, for example by improving navigation, reducing flood risk, generating power and powering mills. However, these changes have created long term ecological damage to our river systems, their hydrology and the wildlife that call them home.

River dredging and straightening

As a result of heavy dredging and straightening programmes over many decades, the Thame along most of its length is much wider, straighter and deeper than it would be naturally. In some places the river is twice as wide and deep as it should! These modifications in most places were carried out to drain the land for agriculture. Channels are usually straightened to reduce flood risk in a specific area. Water flows faster in a straightened channel so it speeds past and is less likely to flood a vulnerable area, which often just moves the problem from one place to another.

Channel narrowing on the Thame at Waterstock Mill

These changes cause the natural processes of a river system to change. For example the conveyance of the river increases which often slows down the velocity of water, causing more sediment to be deposited which often leads to an increase in weed growth and often a lack of plant diversity.

River restoration such as re – meandering can be carried out to rectify some of these issues but a project to reverse the issues of the Thame would cost millions! Small scale restoration such as adding woody debris or narrowing the river can be done on a smaller scale which we are carrying out around the catchment.

In channel structures and blockages

Habitats which were once continuous have over time, become divided by weirs, sluices and culverts restricting the movements of organisms and separating them from habitats, resources and other individuals. This impacts on survival and the completion of the fish life-cycle. Fragmented habitats also lower genetic variability, due to the restricted effective population size, potentially placing populations at an evolutionary disadvantage. These blockages have a negative impact on river processes in particular sediment transportation.

Fish pass created by the Environment Agency in Cuxham

The River Thame has also been a victim of several pollution incidents over the last few decades and thousands of fish have been killed. Fragmented habitats are less resilient to these pollution incidents and the blockages prevent the fish from moving away from the pollution and being able to re- colonise after these events.

The River Thame Conservation Trust with its partners are working on weir removal, fish by pass projects (where structure removal isn’t an option) and de- culverting of water courses around the river Thame catchment.